The present invention relates to a flow control technique in data transmission/reception between a transmission device and a reception device, and more particularly to a flow control technique of credit base in data transmission/reception between a transmission device and a reception device having a redundant structure.
In general, a flow control in data transmission/reception between a transmission device and a reception device implies an operation of the reception device (reception side) to stop data transmission from the transmission device (transmission side) before the reception side becomes at a reception disable state, when the transmission side transmits data, for example, in a packet form of a fixed length or variable length to the reception side.
As kinds of the flow control, there are: a flow control of reception base where a state of the reception side (for example, a data accumulation state) is self-monitored, and the reception side notifies the transmission side of a data reception enable state or data reception disable state by using a resumption signal or transient stop (wait) signal (Rdy/Full, XON/XOFF, or the like); and a flow control of transmission base such as a credit base flow control, to which the present invention relates.
In the case of the flow control of the reception base (refer to FIG. 1), the reception enable state or reception disable state of the reception side is notified to the transmission side through a different line (a line different from a packet transmission route) or through in-band transmission (any field of a packet).
In this flow control, a state of the reception side shows that data can be transmitted from the transmission side or not. As long as any report is continuously transmitted from the reception side, the flow control functions without any problem in spite of the employment of the redundant structure. Also, since the operation is also simple, this flow control is often used in a data transmission device such as a router and a switchboard.
However, in the flow control of the reception base, the reception side needs to self-monitor and control a threshold of a buffer (buffer memory, Buff). Thus, the reception side needs to include a buffer having a memory capacity large enough therefore, and the latency of a state report is also required to be considered as the buffer endurance. Hence, an increase in the memory capacity of the buffer cannot be avoided.
On the other hand, in the flow control of the transmission base, to which the present invention relates, namely, in the credit base flow control (FIG. 2), the control is performed such that, each time the transmission side carries out data transmission, a credit value is subtracted by “1”, and when the credit value becomes “0”, the data transmission is not executed any longer. The reception side instructs the transmission side to carry out the operation of a credit update and add “1” to the credit value, when the process of data transmitted from the transmission side is ended.
In this flow control, since the credit value of the transmission side is set and controlled, a buffer amount in the reception side can be decreased. Also, the transmission side can easily limit on traffic. Thus, there is a merit where the compatibility with an application of a band control is high.
However, unlike the flow control of the reception base, the credit value is required to be managed, which makes the control complex. When the credit value is used between the transmission and reception devices employing a redundant structure, at a time of system switching, the credit values of the reception devices may be different depending on the degree of reading data from the buffer in each system. Because of such problems, the credit value is not applied to the flow control of a redundant structure device.
Note that conventionally, in a duplex ATM (Asynchronous Transfer Mode) switch and the like, there are techniques for switching a system without generating a cell loss. However, they are just proposals of a method of preventing the cell loss.
Here, assuming that the credit base flow control is applied to an edge router serving as a data transmission device of N (N=2) multiplexed configuration, a cell as a fixed length packet is copied from a traffic manager side serving as the transmission side to a main switch side of the N-multiplexed configuration serving as the reception side.
At this time, the cell transmission situation is different between an ACT system (active system) and an SBY system (preparatory system/standby system) due to the congestion state of the cell inside the main switch, a frequency error of an oscillator in each main switch, and the like.
This results in a difference in the credit value between the ACT system main switch and the SBY system main switch. However, when the system switching occurs in this condition, since a credit value of the traffic manager is originally synchronous with a credit value of an old ACT system main switch, a mismatching with a credit value of a new ACT system main switch is induced. The mismatching with this credit value brings about problems of the drop in the throughput inside the main switch after the system switching, and the overflow or underflow of a FIFO (First in First out) buffer memory.
The following are related arts to the present invention.
[Patent Document 1]
                Japanese Patent Laid-Open Publication No. 03-128547[Patent Document 2]        Japanese Patent Laid-Open Publication No. 05-56065        